ABSTRACT Viridans group streptococci (VGS), especially Streptococcus mitis, are pivotal pathogens in a variety of invasive endovascular infections,including: i) ?breakthrough bacteremias? and ?toxic shock? in neutropenic cancer patients; and ii) infective endocarditis (IE). Given world-wide trends in penicillin-resistance and other ?-lactam MIC ?creeps? amongst S. mitis strains, the proportion of serious infections caused by relatively or fully ?-lactam-resistant-(R) strains is disturbing. Moreover, clinical outcomes in such cases utilizing alternate regimens (e.g., vancomycin) have been disappointing, presumably related to the high prevalence of vancomycin ?tolerance? in such strains. This has prompted use of newer bactericidal agents, like daptomycin (DAP) for severe S. mitis syndromes in strains with ?-lactam-resistance. Alarmingly, recent recognition of rapid, durable and high-level DAP-R induced by DAP therapy has significantly reduced enthusiasm for such approaches. In addition, DAP MICs in the S. mitis group are 2-10-fold higher than all other VGS groups. The number of reported clinical cases of invasive S. mitis infections in which DAP-R has emerged is limited, due to the relatively infrequent use of DAP in such infections to-date. However, progressive rise in S. mitis ?-lactam-R plus the inconsistent outcomes of VANC therapy in such syndromes virtually assures increased DAP use, leading to DAP-R S. mitis infections. Moreover, medical centers with high DAP usage have recently confirmed substantial MIC ?creeps? amongst enterococci. Understanding mechanism(s) of emergence of DAP-R in S. mitis, plus strategies to circumvent its evolution are, thus, of great clinical significance. Our Preliminary Data showed that DAP-R outcomes in S. mitis are likely to be multifactorial on both phenotypic and genotypic levels. Most interestingly, we have now shown compelling evidence of two forms of ?DAP hyperaccumulation? in which individual cells in a given streptococcal chain can hyper-capture DAP and either die (?altruistic suicide?) or resist DAP killing, in order to protect the remainder of the cell population from DAP exposures and lethality. This is an apparently unique mechanism of DAP-R amongst gram-positive pathogens. In this proposal, we will use strategic fluorescence microscopy, flow cytometry with multidimensional physiologic interrogations, and single cell sorting plus genotyping to divulge mechanisms by which DAP-R S. mitis can resist DAP exposures. Finally, we will use two well-characterized models of endovascular infections, ex vivo (chamber model) and in vivo (experimental rabbit IE), to define DAP regimens to both circumvent emergence of DAP-R and enhance clearance of S. mitis. In summary, these studies will divulge clinical strategies to forestall emergence of DAP-R in S. mitis and perhaps other Gram-positive pathogens.